Conventionally, power transmission lines with ratings ranging from medium to ultra high voltage (e.g., up to 1.2 MV or higher), use porcelain insulator assemblies to mechanically support and isolate overhead voltage lines. The assemblies may incorporate instrument transformer components. Although porcelain insulators are used in lower voltage (1 kV to 100 kV applications) the structural design considerations can differ substantially for higher voltage applications, in part because of the large physical size and increased mass of the higher voltage insulator assemblies. These assemblies are large vertical structures, essentially towers, which may extend twenty meters or more above the ground, requiring structural designs which assure enduring mechanical integrity and stability.
Porcelain insulator assemblies that operate in the high voltage regime are relatively massive structures available in numerous designs to perform a variety of functions. These functions include provision of instrument transformers or means for isolated connections to power transformers which step the voltage up or down by orders of magnitude. Generally, these such assemblies are elongate, vertically mounted structures comprising a hollow or solid glazed porcelain body having first and second open ends. The ceramic body may have a length dimension along which it extends three meters or more in height when erected above a ground plane but, more commonly, may have a length dimension extending in the range of two meters. Multiple porcelain insulator bodies are at times interconnected (end to end) to create a larger structure on the order of 15 meters in height or even taller. Typically, the insulator bodies are mounted on pedestals which may range from three to seven meters in height. Depending on the voltage rating, the larger complete structures may weigh on the order of 600 Kg or more, with individual porcelain insulator assemblies weighing about 100 Kg. Typically the fastening point between an insulator body and the pedestal is a joint subject to a significant moment. The forces encountered are especially large under wind loading because wind load typically increases as a function of height above the ground plane. Unavoidably, stresses placed on the mounting joints, that connect the relatively heavy porcelain bodies to one another or to a mounting pedestal, undergo micro movements.
A common feature of these insulator assemblies is provision of a metal attachment flange as the joint serving as a transition element from an end of a vertically oriented porcelain body to another structure. The flange interfaces a ceramic surface with a metal system to provide structural integrity to the entire assembly. The term flange, as used herein refers to a collar or a ring-shaped structure attachable to a surface, such as a metal plate, and having an opening through which a member can be inserted and attached for mounting the member to the attachable surface. In the context of the present invention, the flange opening receives and secures an end of the porcelain body and the flange is securely connected to another structure, such as the support pedestal or the high voltage line. However, an attachment flange may connect either end of the porcelain body to another porcelain body or to an intervening assembly such as a housing containing electrically active components, where the housing is positioned between porcelain bodies or between a porcelain body and a pedestal. Such intervening assemblies may be low voltage or provide current connections to equipment performing monitoring functions. In numerous applications, the attachment flange may be integrally formed with a mounting plate which can be bolted to an underlying structure such as a housing containing electrically active components. Generally, the flange serves as a transition element from a lower end of the vertically oriented porcelain body to a structural member such as the surface of the pedestal for stability.
Similarly, another attachment flange, serving as a transition element, may provide means for securing an upper end of a vertically oriented porcelain body. When multiple porcelain insulator bodies are interconnected, the connections between bodies may also be effected with pairs of connected flanges. Because of their size and the weight of these insulator assemblies, the joint between the porcelain body and the metal flange must exhibit substantial mechanical strength, especially when the structure is mounted in an outdoor environment where it may be exposed to large fluctuations in weather conditions, including wind loading, freeze-thaw cycles, or large temperature variations.
In the past, the need to provide a mechanically stable interface between the metallic and ceramic surfaces under substantial load conditions has been met with application of cement grout between mating surfaces. Generally the cement acts as a locking medium to keep flanges of the metal plates attached to the porcelain insulators. In one series of designs the mating surfaces each have features such as surface roughness or machined grooves and the cement grout extends into the surface features to provide a lock which secures the position of the porcelain end within the flange. Because the cement grout has desirable mechanical properties, but cannot bond to either of the mating surfaces, this locking arrangement has been relied upon to limit the extent to which each surface can move relative to the other surface. The cement grout normally fills all voids within the interface between the surfaces to maximize the mechanical strength of the joint being formed.
The size of the flange, the thickness of the gap between mating surfaces, and the volume of cement applied are a function of the required mechanical strength for the joint. A conventional method for attaching the flange of a metal connecting plate to the end of a porcelain body with cement grout normally includes the following steps:    1. Forming sand bands along the portion of the porcelain body which faces and mates with the metal surface, and forming relatively deep grooves along the metal surface of the flange which faces the sand bands.    2. Applying a coating material along the sand band to serve as a gasket coming into contact with the cement, as well as a cushion which compensates for effects of thermal expansion.    3. Applying a coating material along the metal surface to inhibit corrosion and to act as a second gasket as well as a cushion which compensates for effects of thermal expansion along the portion of the surface which comes into contact with the cement.    4. Bringing the surfaces together in a mating fashion with a gap between the surfaces.    5. Filling the gap with grout cement to eliminate voids. The voids being filled with the grout cement may range in width (between grooves formed along the metal surface) from 6 mm to 25 mm, or depth (based in part on groove depth) from 25 mm to 381 mm.
A critical feature of this process is formation of a specially blended grout with limitations in the size of aggregate particles. Otherwise, the grout would not be effective for completely filling small voids or gaps. It has been determined that small variations in the blending and mixing process for the grout can result in substantial degradation in mechanical strength of the resulting joint and, thus, premature failure. In fact, when mechanical strength is compromised by, for example, including too much water in the mixture, failures in the joints of such structures are known to result when the joints are subjected to freeze-thaw cycles, seismic events, wind loading, static mechanical loads or dynamic mechanical loads. Further, a lengthy curing process characteristic of Portland cement products is needed to assure integrity of the grout joint. A one week period is typically required for a sufficient partial cure, after which the joint is strong enough to tolerate modest in order to continue the manufacturing process.
A period of about one month is needed to assure a complete cure. If the assembly is moved prematurely, or if partially cured units are exposed to an excessively dry environment, the mechanical strength of the cement joint can be compromised. Similarly, use of grout which is stored in an unsuitable environment, or for too long prior to use, can also result in inferior mechanical strength.
Like reference characters refer to the same or similar parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead having been placed on illustrating principles of the invention.